Preparation of gap-si heterojunction by liquid phase epitaxy

ABSTRACT

A SATURATED MOLTEN SOLUTION OF GAP AND SI IN A SITUABLE SOLVENT (PB,SN) WAS PREPARED. THE MOLTEN, SATURATED SOLUTION WAS BROUGHT INTO CONTACT WITH A SI SINGLE CRYSTAL SUBSTRATE AND COOLED, CAUSING THE GAP TO FORM AN EPITAXIAL LAYER ON THE SI SURFACE. THE CRYSTAL LATTICE STRUCTURE AND DIMENSIONS OF SI AND GAP CRYSTALS ARE VERY SIMILAR. THE PREFERRED PHYSICAL PROPERTIES OF THE SOLVENT ARE: A RELATIVELY HIGH SOLUBILITY FOR GAP; A RELATIVELY LOW SOLUBILITY FOR SI; AND A RELATIVELY LOW SOLUBILITY OF THE SELECTED SOLVENT IN THE EPITAXIALLY GROWN GAP.

May 14, 1914 TEMPERATURE G. A.A YPAS ETAL 3.3%,794 PREPARATION OF GAP-HETEROJUNCTION UY LlQUJD PHASE EPITAXY Filed Sept. 24, 1970 TILTABLETILT TOGROW POSITION I E I RETURN TO E" I ig fi'w GEORKQEVAEEZ$PAS 1 IFERENC eaqszmcw EIEHGIQDEO NE EMY I l D E United States Patent O US. Cl.148172 9 Claims ABSTRACT OF THE DISCLOSURE A saturated molten solutionof GaP and Si in a suitable solvent (Pb, Sn) was prepared. The molten,saturated solution was brought into contact with a Si single crystalsubstrate and cooled, causing the GaP to form an epitaxial layer on theSi surface. The crystal lattice structure and dimensions of Si and GaPcrystals are very similar. The preferred physical properties of thesolvent are: a relatively high solubility for GaP; a relatively lowsolubility for Si; and a relatively low solubility of the selectedsolvent in the epitaxially grown GaP.

FIELD OF THE INVENTION This invention relates to the epitaxial growth ofGaP on Si by liquid phase epitaxy and more particularly to the use of Pband/or Sn as solvents for that purpose.

DESCRIPTION OF THE PRIOR ART It is known to epitaxially deposit GaP ontoSi by fused salt electrolysis as reported by I. J. Cuomo and R. J.Gambino, J. Electrochem. Soc: Vol. 115, page 755 (July 1968). Further,liquid phase epitaxy has been employed to produce heterojunctions suchas Ge-Si as reported by K. Kurata and T. Hirai, J. Electrochem. Soc.:Vol. 115, page 869 (August 1968), and Ge on GaAs as reported by F.Rosztoczy, J. Electrochem. Soc: Vol. 115, page 3280 (1968).

Gas phase heteroepitaxy has also been reported by Robert W. Thomas, J.Electrochem. Soc: Vol. 116, No. 10, (1969) wherein epitaxy of GaP onsingle crystal silicon was achieved using gallium and phosphorousalkyls.

Further hereteroepitaxial growth of GaP on single crystal silicon wasachieved by Osamu Igaroshi as reported in Journal of Applied Physics,Vol. 41, page 3190 (1970), This technique involved the evaporation ofthe compound onto a suitably prepared silicon substrate.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWING Furtherobjects and advantages of the present invention and the method ofgrowing GaP on Si will become apparent from the following detaileddescription taken in conjunction with the, drawings in which:

FIG. 1 is a sectional view of a tiltable furnace tube in the standbyposition showing the saturated solution and the silicon substrate beforecontact;

FIG. 2 is a sectional view of the tiltable furnace in the growingposition showing the saturated solution covering the Si substrate duringepitaxy; and

FIG. 3 is a plot of temperature versus time depicting the thermal cycleemployed by the apparatus of FIG. 1 and 2.

Referring to FIG. 1, there is shown a tiltable furnace tube 10 in afirst or standby position, containing a boat 12.

Boat 12 is made of a refractory material, for example,

graphite, BN, or quartz. A solution 14 of GaP and Si in a solvent isshown at the lower end of boat 12. The solution is preferably saturatedwith GaP to expedite epitaxial growth later. Solution 14 is maintainedin the molten state by a furnace (not shown). Excess GaP 15 on solution14 assures that the solution is saturated. A Si wafer or substrate 16 ismounted at the upper end of boat 12 by means of a refractory hold downscrew 18 which is also a refractory material. It is preferable that Sisubstrate 16 be a single crystal having a crystal orientation of (111)or A flow of inert gas, preferably H is maintained through furnace tube10 to prevent oxidation of molten solution 14. Contact between solution14 and substrate 16 is established by tilting furnace tube 10 into asecond or growing position as shown in FIG. 2. Solution 14 rolls to theopposite end of furnace tube 10' and covers substrate 16 causing a GaPcrystal 20 to form. The apparatus of FIG. 1 is similar in design to theH. Nelson apparatus described in RCSA Review, vol. 24, page 603 (1963).Clearly other types of solution-substrate contacting apparatus may beemployed.

FIG. 3 depicts the thermal cycle employed in growing GaP layer 20 onsubstrate 16 using the apparatus of FIG. 1. From t to t the temperatureis elevated with furnace tube 10 in the standby position. From time I,to time t furnace is maintained at a preselected constant temperatureThigh to melt the initial charge and form saturated solution 14. At timefurnace tube 10 is tilted into the growth position and solution 14 rollsacross boat 12 and covers substrate 16. The temperature is slowlydecreased to T causing GaP to form an epitaxial layer onto substrate 16.The lattice constant of GaP is 5.4505 and the lattice constant of Si is5.43072, sufficiently matched to permit GaP to epitaxially grow on thesurface of Si substrate 16. At time t furnace tube 10 is tilted back tothe standby position causing solution 14 to return to its originalposition. The excess solution is removed from the surface of substrate16 by a scraper apparatus (not shown).

The solvent or carrier material in saturated solution 14 may be formedby more than one material, and is selected on the basis of the followingphysical properties:

(1) Relatively low solubility coefl-lcient for -Si.-This limits theamount of Si that dissolves into the solution 14. In this way the dopingand other effects of silicon can be minimized.

(l) Relatively high solubility coefficient for GaP.-- This permits theGaP to go into solution for epitaxial growth onto the substrate.

(2) Relatively high solubility coefiicient for GaP.--This permits theGa? to go into solution for epitaxial growth onto the substrate.

(3) A low solubility coefiicient into the GaP crystal which is beinggrown.-This limits the concentration of solvent included in the GaPcrystal which minimized the doping effect of the solvent.

(4') Preferably removable by cleaning solvent such as H01 and HNO.--After the GaP-Si junction has been prepared, it may be desirable toclean the Ga? surface to remove any carrier solvent which was notremoved by scraping molten solution 14 in furnace tube 10.

Pb solvent embodiment Pb is a suitable solvent for the presenttechnique. Pb has a low solubility coefficient for Si (about 0.1 molpercent at 950 C.) and a high solubility coeflicient of GaP (about 2 molpercent at 950 C.). In this embodiment saturated solution 14 wasprepared by thoroughly mixing 13 grams of Pb and 0.3 gram of GaP. Theamount of GaP is more than can be dissolved in 13 grams of Pb at theThigh involved which guaranteed that the solution was saturated. Themixture Was placed in boat 12 and maintained at 950 C. for 60 minutes. Asmall portion of the Ga? remained undissolved. The system was quenchedand subsequently saturated with Si at 950 C. for 60 minutes to preventSi substrate 16 from dissolving into solution 14 during the FIG. 2contact step. The system was quenched again and Si substrate 16 wasplaced in boat 12 at the opposite end thereof from saturated solution14. The quenching steps are not shown in FIG. 3. The temperature of thesystem was raised to 950 C. (T and maintained for 30 minutes. Furnacetube 10 was tilted into the growing position and solution 14 came incontact with substrate 16. The temperature was dropped to 800 C. (T overa period of 60 minutes. Furnace tube 10 was tilted back to the standbyposition. The resulting epitaxial layer 20 was approximately 10 micronsthick.

Thigh (in this case preferably 950 C.) is not critical and is a functionof the solubility coefficients of the materials and the desiredcomposition of solution 14. Preferably solution 14 should be about 1% byweight of GaP. The concentration of GaP can be as low as .1% by weight,in which case the epitaxial growth proceeds very slowly. Theconcentration of GaP may be as high as 20% or even higher. At the highGaP concentrations, the epitaxial growth proceeds rapidly causingdefects in the crystal lattice and incorporation of impurities from thesolution. T (in this case preferably 800 C.) was selected on the basisof the required thickness of GaP layer 20. Slower cooling rates tend toproduce higher quality crystals, and of course avoid the possibility ofconstitutional super cooling. Subject to these considerations, thetemperatures and cooling rate are not critical.

The time periods described above are representative of the time requiredfor the solution to reach equilibrium without external agitation. Atomicdiffusion was the primary means through which equilibrium was obtained.The time periods may be shortened considerably by agitating solution 14or rocking boat 12. In any event the time periods are not generallycritical.

Sn solvent embodiment Sn is also a suitable solvent having a relativelylow solubility coefiicient for Si and a relatively high solubilitycoefiicient of GaP. A saturated solution of GaP in Sn was prepared byplacing grams of Sn and 0.5 gram of GaP in boat 12 (more than sufiicientGaP for the temperature involved) and maintaining the temperature at 850C. for a period of 60 minutes. Solution 14 was quenched and Si saturatedas described in the Pb embodiment and brought into contact withsubstrate 16. The temperature was lowered to 650 C. over a period of 3hours, resulting in a micron GaP layer epitaxially grown on Si.

If desired, the quenching steps can be eliminated by initially placingall of solution 14 constituents (GaP, Pb and/or Sn, Si) at the lower endof the boat in the standby position. Si substrate 16 is then mounted atthe upper end of boat 12. The temperature of the system is cycled onlyonce as depicted in FIG. 3.

The resulting GaP-Si heterojunction has a low cost Si base. The price ofthe Si substrate is approximately 1000 4 times less than the price ofsuitable GaP substrates. The uncoated side of the Si substrate is alsosuitable for use as part of the integrated circuit employed inconjunction with the GaP layer.

Clearly various changes may be made in the structude and embodimentsshown herein without departing from the concept of the presentinvention. In addition to the horizontal apparatus of FIGS. 1 and 2, avertical structure may be employed wherein the substrate is dipped intothe solution (see F. E. -Rosztoczy, Varian Technical Journal, No. 3,page 1 (Spring 1970). Further a steady state-temperature gradienttechnique may be employed which does not employ a thermal cycle asdepicted in FIG. 3. Instead, the substrate is maintained at a constanttemperature slightly lower than the source of GaP causing a gradientacross the solution.

What is claimed is:

1. In a method for forming a binary GaP layer on a Si substrate throughliquid phase epitaxy comprising the steps of:

providing a solution of GaP in a solvent at a first temperature whereinthe solvent is at least one element selected from the group consistingof Pb and Sn; providing a suitable Si substrate;

establishing contact between the Si substrate and the solution; and

causing the Ga? in solution to epitaxially form binary GaP over the Sisubstrate by lowering the temperature of at least that part of thesolution proximate the substrate to a second temperature which is lowerthan the first temperature.

2. The method of claim 1 wherein the solvent is Pb.

3. The method of claim 2 wherein the first temperature is 950 C. and thesecond temperature is 800 C.

4. The method of claim 1 wherein the solvent is Sn.

5. The method of claim 4 wherein the first temperature is 850 C. and thesecond temperature is 650 C.

6. The method of claim 1 wherein the solution of GaP and solvent issaturated with GaP and the Si substrate is a single Si crystal.

7. The method of claim 6 wherein the solution of GaP and solvent issaturated with Si before contact with the Si substrate.

8. The method of claim 6 wherein the concentration by weight of GaP inthe solutionis from about 0.1% to about 20%.

9. The method of claim 6 wherein the concentration by weight of GaP inthe solution is about 1%.

References Cited UNITED STATES PATENTS 3,278,342 10/1966 John et al1481.6

OTHER REFERENCES Woodall, Growth of Ga in P on Si substrates, IBMTechnical Disclosure Bulletin, vol. 12, No. 11, April 1970I p. 1834.

J. Electrochem. Soc., vol. 115, pp. 755-9, July 1968.

J. Electrochem. Soc., vol. 116, No. 10, pp. 1449-50.

Journal of Applied Physics, vol. 41, pp. 3190-2 (1970).

GEORGE T. OZAKI, Primary Examiner U.S. c1. X.R. 14s 171, 173; 117-201 5

